GBPPR Non-Linear Junction Detector

Overview

A Non-Linear Junction Detector (NLJD) is a counter-surveillance tool commonly used for detecting hidden transmitters or other electronic items. They work by transmitting a clean (no harmonics or audio modulation) 900 MHz RF signal at the target location and displaying the received signal strengths of the detected second (1800 MHz) and third (2700 MHz) harmonics. By comparing the received signal strengths of these two harmonics, the operator can distinguish if the target location contains a dissimilar metal non-linear junction, such as some rusty nails, or an actual P-N junction, such as a diode or transistor.

A "P-N junction" is the physical boundary area between the P-type and N-type semiconductor material used in the creation of our modern electrical components. By their nature, these boundary areas are "non-linear," meaning that when illuminated with a RF carrier they will generate successive harmonics of that initial illumination carrier signal. This harmonic generation is a function of the physical construction of the P-type and N-type boundary areas and doesn't even require the item to be powered on. In nature, a similar "P-N junction" is often created between two dissimilar metals at a catalyst. You've probably seen this in action as rust eating away on anything you've left outside for awhile. You can differentiate between a true semiconductor P-N junction and a "nature junction" by comparing the signal received signal strengths of the different harmonics generated. True semiconductor P-N junctions tend to generate strong even harmonics (2nd, 4th, 6th, etc.), while dissimilar metals tend to create strong odd harmonics (3rd, 5th, 7th, etc.). Also, a harmonic signal from a true semiconductor P-N junction will be "quiet" when audio demodulated. Since the illumination RF carrier is clean and unmodulated, those even-order harmonics will also be clean and unmodulated. Compare this to the odd-order harmonics from dissimilar metals, which will tend to be "noisy" or "scratchy" when audio demodulated. If you've located a suspect area with your NLJD and hear "crackling noises" as you lighty pound around the area with a rubber mallet - you can be pretty sure it's just a dissimilar metal junction.

That's the idea at least, as I have no idea if the final project will work... The NLJD described here will be a bit clunky, but should be a good starting point for something more easily constructable and useable in the "real-world." The entire project will be built and documented as individual modules over a series of articles. This will allow for time to test and develop all the components for the project.

The main concept in a radio project of this type is isolation. You basically have a radio transmitter putting out around +30 dBm right next to a receiver trying to detect a harmonic which may be below -100 dBm. The use of well-shielded module boxes, double-shielded coax, and high-quality RF connectors is highly recommended. I prefer using old California Amplifier MMDS downconverter cases. These cases are very well constructed, provide threaded holes (3/8"-32) for standard RF connectors, and can be had at very low cost - if you can find them! Most of the other parts for this project were scrouged from other electronic devices and surplus radio equipment found at ham radio swapfests, so some of the components may be rare or hard to find.

Feed-through capacitors should be used to route any non-RF signals in-or-out of the module boxes. This will prevent any excess leakage or RF interference. Voltage conditioning and regulation is not shown in the schematics, as it's all pretty standard. Try to use those new low-noise voltage regulators, though. Circuits with a PLL and VCO should each have their own dedicated voltage regulator to minimize interaction between them. Components in the pictures may vary from the schematics due to tweaking, but the schematics will have the correct values.

Using an expensive TCXO like this is a bit of overkill, as just a regular 10 MHz crystal oscillator circuit will also work. Since both the transmitter and receiver frequency synthesizers will share the same time base, any frequency drift in the reference crystal should also track between those two circuits.

A simple 2N2222A transistor and 74AC00 buffer the 1 volt peak-to-peak output from the TCXO and convert it into a clean TTL-compatible square wave.

This may turn out to be a problem, as the harmonics generated by this time base extend all the way into the microwave spectrum. Without proper shielding, the time base could generate spurs on the frequency you wish to monitor - essentially jamming itself!

A filtered sine wave-based 10 MHz TCXO time base is currently under development. I'm also about 80% certain there is no need to even buffer the output from the TCXO, as most of the PLL synthesizer ICs have high-impedance reference frequency inputs.

The 10 MHz time base with have three reference outputs, provided via standard F jacks. One is for the transmitter, one is for the first local oscillator, and the other is for the second local oscillator.

Consider this time base design experimental, for now.

Overview of the second Local Oscillator (LO) and mixer.

This circuit converts the 436 MHz first Intermediate Frequency (IF) down to the 9 MHz second IF. It does this by mixing the incoming 436 MHz RF signal with a local oscillator frequency of 445 MHz. The mixing takes place in a Mini-Circuits SBL-1 mixer.

A high-dynamic range post-mixer 9 MHz IF amplifier helps to recover some of the power lossed through the mixer. It also provides the first bit of final filtering via the low-pass diplexer circuit.

The yellow toroid is approximately 0.8 µH and consists of 15 turns of #30 enameled magnet wire on a T-25-6 powered-iron core.

The other toroid forms a 4:1 matching transformer to convert the 200 ohm output impedance of the 2N5109 down to 50 ohms. It consists of ten bifilar (twisted together) turns of #28 enameled magnet wire on a FT-37-43 ferrite core. Each winding measured around 0.42 µH. Be sure to keep track of the phasing when winding the core.

Here's a list of spurs (under 1 GHz) generated by this circuit with a -30 dBm input at 436 MHz:

In order to reduce the height of the Mini-Circuits SBL-1 mixer and 2N5109 transistor, the PC board was drilled to pass the component's leads through. Then little extension pads where added to connect back up to the top side of the board via small connecting wires.

This also helps to give the final circuit additional isolation as any RF leakage is trapped underneath the circuit board.

The Crystek CVCO55CL-0393-0428 isn't ideal for this circuit, as its stock RF output power is a little low (+3 dBm), but I had one available from another project and they are available from Mouser. We'll run the VCO at +6 VDC in order to help bump up the output RF power a few dBms.

The components which make up the PLL loop filter itself should be of high-quality and low-leakage to minimize the generation of microphonics or excess carrier sideband noise. Try to use 1% metal-film resistors and non-polarized polystyrene or other film capacitors.

A 3 or 6 dB resistive attenuator pad should be added to the 9 MHz output of this circuit to help the mixer and 2N5109 "see" a 50 ohm impedance. This also helps to tame the high-impedance of the resolution filter down line.

Spectrum analyzer view of the 9 MHz second IF output with a -30 dBm 436 MHz RF input signal.

Overview of the 9 MHz resolution filter, 9 MHz final IF amplifier, and the logarithmic detector.

The 9 MHz input from the second mixer is on the upper-left and is split via an optional Mini-Circuits PSC-2-1 RF splitter. One of the ports on the splitter continues onto the resolution filter and the other will go to a front-panel BNC connector for external processing.

The 9 MHz resolution filter shown here is a KVG XF-9M crystal filter with a bandwidth of 500 Hz. These filters were all the rage in amateur radio about 30 years ago, and may be difficult to find today. There are homebrew drop-in replacements for 9 MHz filters with the same basic specifications available from time-to-time on eBay.

The final Received Signal Strength Indicator (RSSI) voltage output from the Analog Devices AD8307 is on the bottom-left via a BNC jack.

The output from the resolution filter is then sent to a combination Analog Devices AD603 / AD8307 IF amplifier and logarithmic detector. This circuit is straight from the AD8307's datasheet (Figure 40 - 120 dB Measurement System), with only a few minor tweaks.

It's total overkill, but the AD603 / AD8307 combination work amazingly well together for only around $20 in parts. Most commercial (and government) NLJD IF/log detector strips are based around common FM receiver ICs and tend to have a poor dynamic range.

These values are subject to change as I fiddle with the overall design. The impedance matching to the KVG filter may need a little tweaking, which will effect the final value. The actual voltage level isn't important, just that it changes properly as the input RF power level also changes.